Many analytical or industrial processes require the generation of beams of ions of particular substances or analytes. For example, ion beams might occur in ion guns, ion implanters, ion thrusters for attitude control of satellites, laser ablation plumes, and various mass spectrometers (MS), including linear quadrupole MS, quadrupole ion trap MS (e.g. "Paul" trap), ion cyclotron resonance MS, time-of-flight MS, and electric and/or magnetic sector MS. Several schemes are known in the art for generating such ion beams including electron impact, laser irradiation, electrospray, and variations thereof such as lonspray.TM., thermospray, inductively coupled plasma sources, glow discharges and hollow cathode discharges. Typical arrangements combine a sample with a carrier or support gas whereby the carrier gas is utilized to aid in transporting, ionizing, or both transporting and ionizing, the sample. As is well known, analyte substances often occur in combination with other substances which can also be ionized and transported along with the analyte ions and carrier gas ions. These other substances will be collectively referred to as matrix substances, or in ionized form as matrix ions. The combined form of analyte and matrix will be referred to as the sample, or in ionized form as sample ions. Thus, matrix refers to all substances in a sample apart from the analyte. Similarly, matrix ions refers to all ions apart from the analyte ions and thus, matrix ions includes plasma ions. For example, when an aqueous nitric acid sample is introduced into an argon ICP, plasma ions such as ArH.sup.+, ArO.sup.+, ArN.sup.+ are formed. The matrix ions are typically an interference in chemical analysis applications or other utilizations of the plasma or ion beam.
For example, in a typical arrangement a sample is combined with the carrier gas in an electrical field, whereupon the sample and the carrier gas are ionized in a strong electric or magnetic field and later used in an analytical or other process. In another typical arrangement, the carrier gas is first ionized in a strong electric or magnetic field whereupon the sample is then introduced into the ionized carrier gas. Ionized carrier gas contains carrier gas and carrier gas ions. Electric fields are generated by a variety of methods well known in the art including, but not limited to, capacitive and inductive coupling of radiofrequency (RF) and/or DC electrical energy.
In an inductive coupling arrangement, an RF voltage is applied to a coil of a conducting material, typically brass. In the interior of the coil, one or more tubes supply a carrier gas, such as argon, and a sample, which may be any substance or mixture of substances. The analyte may be supplied in a variety of forms including but not limited to a gaseous form, as a liquid, as a droplet form, as an aerosol, or as a laser ablated plume. A large electrical field is generated within the coil. Within this field, any free electrons will initiate a chain reaction in the sample and the carrier gas causing a loss of electrons and thus ionization of the carrier gas and the sample. Several methods well known in the art, including but not limited to the introduction of a Tesla coil, the introduction of a graphite rod, or thermal emission of electrons, will provide free electrons causing initiation of a chain reaction. The result is an ionized gas or plasma consisting of both free electrons and charged and uncharged species of the carrier gas and the sample. The species of both the carrier gas and the sample in the plasma may be in the form of particles, atoms or molecules, or a mixture of particles, atoms and molecules, depending on the particular species selected for use as the carrier gas and the species and form selected for use as the sample.
The carrier gas and the sample may be combined by a wide variety of methods well known in the art. For example, as described above, the sample in an aerosol form is combined with the carrier gas and directed to the interior of a coil in an inductively coupled plasma. Another typical arrangement, known in the art as electrospray (and variations thereof such as ionspray.TM.), employs a needle which receives a liquid sample from a source such as a liquid chromatograph. Surrounding the needle is a tube which supplies a carrier gas such as nitrogen as a high velocity atomizing carrier gas. Both the needle and the tube empty into a chamber. Upon discharge from the needle, the sample liquid is evaporated and atomized in the nitrogen carrier gas. Ions of both the evaporated liquid sample and the nitrogen carrier gas are produced by creating an electric field within the chamber. The electric field may be produced by creating a voltage difference between the needle and the chamber. A voltage difference may be created by applying a voltage to the needle and grounding the chamber.
The resultant plasma generated by any of the foregoing methods is typically directed towards either an analytical apparatus or towards a reaction zone wherein the carrier gas and sample ions are analyzed or otherwise reacted or utilized in some fashion. The resultant plasma is typically directed by means of an electric or magnetic field, or by means of a pressure differential, or both. As the plasma is directed, the plasma is converted from a plasma to an ion beam. As used herein, the term "ion beam" refers to a stream consisting primarily of positively charged and neutral species. The bulk of the negatively charged species in the plasma are typically electrons, which are rapidly dispersed as the plasma is directed by either electric or magnetic fields or by a pressure differential. However, even after significant dispersal of the ion beam, the ion beam may not be completely void of negatively charged species. As the plasma progresses forward, the free electrons, due to their low mass relative to the positively charged ions, tend to disperse from the plasma, thus converting the plasma to an ion beam. Also, the ion beam itself will tend to disperse due to several effects. Most prominent among these effects are free jet expansion and the repulsive forces of charged species within the ion beam. The effect of dispersion of the constituent species in the ion beam is charge separation among those species and is well known in the art. The resultant ion beam is thus typically characterized by high net positive charge density. Since the carrier gas is typically present in excess over the sample, this high positive charge density is primarily attributable to the relatively high abundance of positively charged carrier gas ions.
In many applications, the abundance of positively charged carrier gas ions, matrix io ions and/or the resultant high charge density may be undesirable. For example, it is often desirable that the ion beam be focused through a small aperture, for example, if the sample ions were to be analyzed in a mass spectrometer. In such an arrangement, where the ion beam is directed through an aperture, the high charge density will prescribe a space charge limit to the maximum on beam current that may be passed through a given aperture. Any beam current in excess of the space charge limit is unable to pass through the aperture and is thus lost. In many applications, the portion of the beam which is lost includes analyte ions. Indeed, a loss of a portion of the beam may result in a disproportionate loss of some or all of the analyte ions because the analyte ions may not be evenly distributed throughout the ion beam or may not respond to the various dispersing and directing forces in the same manner as the carrier gas or matrix ions.
Another example where the presence of carrier gas or matrix ions is undesirable is in a quadrupole ion trap mass spectrometer where the quadrupole ion trap has a limited ion storage capacity. In an ion beam directed into a quadrupole ion trap, the carrier gas and matrix ions compete with analyte ions for the limited storage capacity of the quadrupole ion trap. Thus, to the extent that carrier gas ions or matrix ions can be selectively eliminated from the ion beam, the storage capacity for analyte ions in the quadrupole ion trap is thereby increased. Carrier gas ions and plasma ions can be potent chemical ionization sources and can cause high levels of ionization of background gases in the trap. Such background ions can be formed in sufficient number that they interfere with the detection of analyte ions even if good vacuum practices and high vacuum conditions are maintained. Thus, removal of carrier gas ions and/or plasma ions also has the beneficial effect of reducing such background ionization.
The presence of carrier gas or matrix ions is also undesirable in any application where the analyte ions are to be used in a process or reaction where the carrier gas or matrix ions might interfere with such process. By way of further example, in many integrated circuit manufacturing processes, ion beams may be directed towards a targeted material such as a silicon wafer to impart electrical or physical properties to the material. The desired properties are typically highly dependent on the specific ions directed at such materials. Thus, carrier gas or matrix ions may cause undesirable effects if implanted in the targeted materials.
Thus, in an ion beam having carrier gas ions, analyte ions, and/or matrix ions, there exists a need for a method of selectively eliminating carrier gas ions and/or a portion or all matrix ions without eliminating or neutralizing the analyte ions.
Apparati that have been used to carry out these processes may be characterized as an ion source coupled or connected to a mass analyzer. Specifically known are an inductively coupled plasma ion source connected to a time-of-flight mass spectrometer (Myers et al., Journal of the American Society for Mass Spectrometry, Vol. 6, pp. 411-420 (1995)) or connected to an ion trap (D W Koppenaal, C J Barinaga, and M R Smith, J. Analytical Atomic Spectrometry Vol. 9, pp. 1053-1058 (1994); and C J Barinaga and D W Koppenaal, Rapid Communications in Mass Spectrometry, Vol. 8, pp. 71-76 (1994)), an electrospray ion source connected to a time-of-flight mass spectrometer (AN Verentchikov, W Ens, and KG Standing, Analytical Chemistry Vol. 66, pp. 126-133 (1994)) or to a collision cell containing a non-reactive gas with the collision cell connected to a linear quadropole mass spectrometer (D J Douglas and J B French, Journal of the American Society for Mass Spectrometry Vol. 3, pp. 398-403 (1992); also U.S. Pat. No. 4,963,736]) or an MS ion source through collision cell with non-reactive gas to time-of-flight mass spectrometer (A N Krutchinsky, I V Chernushevich, V Spicer, W Ens, and K G Standing, Proceedings of the 43.sup.rd ASMS Conference on Mass Spectrometry and Allied Topics, p. 126, May 21-26, 1995).
As is known in the art, a collision cell defines a region of space containing a sufficiently high pressure of a gas. The gas may be reactive or non-reactive gas. If necessary, the collision cell is provided with a guiding field (electric or magnetic to ensure that ions can traverse the cell in spite of a large number of collisions with the gas. A typical guiding field may be formed using an RF/DC multipole which restricts ion motion transverse to the long axis of the multipole while allowing relatively unrestricted motion along the axis. Multipoles may be formed in a wide variety of ways well known in the art. Typically, a multipole is formed by arranging an even number of pole elements, typically circular cross section rods, symmetrically around a common axis with the radial separation of the rods constant along the length of the multipole. The theory, design and performance of such multipoles has been described in great detail by Gerlich (Dieter Gerlich, in State-Selected and State-to-State Ion-Molecule Reaction Dynamics, Part 1: Experiment; in Advances in Chemical Physics Series Vol. LXXXII; Cheuk-Yiu Ng and Michael Baer, editors; John Wiley & Sons, 1992) and others.